1. Field of the Invention
Circuits that communicate data may have data inputs for inputting data, data outputs for outputting data, a clock input for timing or synchronizing the data input and/or output communication, and a mode input for controlling the data input and/or output communication.
2. Description of the Related Art
In FIG. 1, a conventional circuit 110 has a data input bus 101, a data output bus 102, clock input bus 103, and mode input bus 104. The circuit 110 responds to the clock input and mode input to either, (1) remain in an idle state where no data communication occurs, or (2) enter a data communication state where data is communicated between the circuit's data input and/or data output.
While the circuit example in FIG. 1 is intentionally simple for clarification, its input/output signaling model, consisting of data input, data output, clock input, and control input signals, could represent more complex circuits. For example the circuit model could represent IEEE 1149.1 test access port circuits implemented in integrated circuits or included in the design layout or data base of intellectual property core circuits, such as CPUs and DSPs, for use as sub-circuits within an integrated circuit. Further, the example circuit model could represent, in general any type, of data communication circuits, such as shift registers, synchronously operated memories, micro-controllers, CPUs, DSPs, analog to digital converters whereby the data input is understood to be analog signal data input, or digital to analog converters whereby the data output is understood to be analog signal data output.
In FIG. 2, the clock signals input on bus 103 time the circuit to operate, in response to mode input on bus 104, in either an idle state 202 or communicate state 204. The circuit 110 will be in the idle state 202 during clocks signals occurring while the mode signal on bus 104 is low, and will transition to the communicate state 204 during a clock signal occurring when the mode signal on bus 104 is high. The circuit will remain in the communicate state 204 during clock signals occurring while the mode signal is high. The circuit will return to the idle state 202 during a clock signal occurring when the mode signal is low.
In the idle state, no data communication occurs in the circuit from the data input and/or data output. In the communicate state, data communication occurs in the circuit 110 from the data input and/or data output. It should be understood that the state diagram of FIG. 2 is intentionally simplified to clarify the description of the invention. A more complex state diagram, having at least an idle state and at least a data communication state could have been used as well. For example, the state diagram of the above mentioned IEEE 1149.1 test access port circuit contains an idle state (RTIDLE) and data communication states (DR-Shift & IR-Shift) and could have been used. However, for the purpose of describing the invention, the FIG. 2 state diagram is adequate.
In FIG. 3, circuit 110 operates according to the state diagram of FIG. 2. In FIG. 3, the circuit 110 remains in the idle state during clock signals occurring while the mode signal is low. The circuit 110 transitions into the communicate state during the first clock signal that occurs after the mode signal goes high. The circuit remains in the communicate state during clocks occurring while the mode signal is high. The circuit transitions back to the idle state during the first clock that occurs after the mode signal goes back low.
The communicate state could operate a circuit as shown in FIG. 1 to: (1) transfer data inputs directly, through an enabled buffer or switch, to data outputs of the circuit; (2) transfer data inputs to the data outputs via intermediate storage circuitry within the circuit; (3) input data to the circuit, process the input data using processing circuitry within the circuit, and output the processed data; (4) input data to the circuit and store the data in a internal memory; (5) output data previously stored in an internal memory; or (6) input and store data while outputting previously stored data.
In this specification, the mode input is evaluated on the rising edge of the clock input to determine state transitions. Also, the clock input will operate as a low to high and high to low pulse that occurs during times when the mode input is in a steady state one or zero logic condition. While a rising edge clock pulse convention is used in this description, a falling edge clock pulse convention could be used as well. Also the mode inputs may be inverted from what is shown in FIG. 3 without departing from the nature of the present invention.